Novobiocin-induced VP-16 accumulation and MRP expression in human leukemia and ovarian carcinoma cells

We have previously reported that novobiocin potentiates the cytotoxic activity of etoposide (VP-16) and teniposide (VM-26) in a number of experimental tumor cell lines by inhibition of the efflux of the epipodophyllotoxins by an ATP-requiring transporter. In leukemia cells from 12/19 patients and in ovarian carcinoma cells from 2/4 patients, novobiocin, in a concentration range of 150-1000 microM, increased the intracellular accumulation of VP-16 by 30-250% by inhibiting its efflux. Novobiocin did not significantly increase the intracellular concentration of VP-16 in human mononuclear bone marrow cells from two individuals with normal bone marrow, suggesting that it might be possible to selectively modulate the intracellular accumulation of the epipodophyllotoxin in tumor cells relative to normal hematopoietic tissue. Previous findings from our laboratory have provided evidence that the membrane transporter for VP-16 which is inhibited by novobiocin is distinct from the P-glycoprotein. The expression of MRP, measured by immunoblotting, was variable in novobiocin-responsive and non-responsive leukemia cells, indicating that no direct relationship existed between the modulatory activity of novobiocin on the transport of VP-16 and the expression of the MRP gene. The findings indicate that the novobiocin-sensitive VP-16 transporter is (i) present in high frequency in leukemia and ovarian carcinoma cells, and (ii) probably not the P-glycoprotein or MRP.

Synthesis and biological evaluation of 1,3-oxathiolane 5-azapyrimidine, 6-azapyrimidine, and fluorosubstituted 3-deazapyrimidine nucleosides

(2R,5S)-5-Amino-2-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]- 1,2,4-triazine-3(2H)-one (8) and (2R,5R)-5-amino-2-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]-1,2,4-tr iazine-3(2H)-one (9) have been synthesized via a multi-step procedure from 6-azauridine. (2R,5S)-4-Amino-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]-1,3, 5-triazine-2(1H)-one (11) and (2R,5R)-4-amino-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]- 1,3,5-triazine-2(1H)-one (12), and the fluorosubstituted 3-deazanucleosides (19-24) have been synthesized by the transglycosylation of (2R,5S)-1-[2-[[(tert-butyldiphenylsilyl) oxy]methyl]-1,3-oxathiolan-5-yl] cytosine (2) with silylated 5-azacytosine and the corresponding silylated fluorosubstituted 3-deazacytosines, respectively, in the presence of trimethylsilyl trifluoromethanesulfonate as the catalyst in anhydrous dichloroethane, followed by deprotection of the blocking groups. These compounds were tested in vitro for cytotoxicity against L1210, B16F10, and CCRF-CEM tumor cell lines and for antiviral activity against HIV-1 and HBV.

Comparison of DNA lesions produced by tumor-inhibitory 1,2-bis(sulfonyl)hydrazines and chloroethylnitrosoureas

1,2-Bis(sulfonyl)hydrazine derivatives, designed to generate several of the electrophilic species classically believed to be responsible for the alkylating (chloroethylating) and/or carbamoylating activities of the chloroethylnitrosoureas (CNUs), were compared with respect to the cross-linking and nicking of T7 DNA to that caused by 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU), and 1-(2-chloroethyl)-3-(4-trans-methylcyclohexyl)-1-nitrosourea (MeCCNU). In the case of BCNU, a large proportion of T7 DNA strand nicking was found to be due to the generation of 2-chloroethylamine, produced from the hydrolysis of 2-chloroethylisocyanate, in turn formed during the decomposition of the parental nitrosourea. 1,2-Bis(methylsulfonyl)-1-(2-chloroethyl)hydrazine (compound 1) gave a greater yield of DNA cross-links than the CNUs. Compound 1, as well as its derivatives that were incapable of generating 2-chloroethylisocyanate, did not produce detectable levels of strand nicking, indicating that N7-alkylation of guanine did not occur to a significant extent with these agents. Since compound 1 and its derivatives are believed to generate chloronium and chloroethyldiazonium ions, it would appear that these species could not be significantly involved in the N7-alkylation of guanine caused by the CNUs. The relatively low level of N7-alkylation of guanine residues and the relatively high yield of cross-links generated by some of the 1,2-bis(sulfonyl)-1-(2-chloroethyl)hydrazine derivatives implies that they are more exclusive O6-guanine chloroethylating agents than the CNUs. O6-Guanine chloroethylation is believed to be the therapeutically relevant event produced by the CNUs; therefore, compound 1 derivatives represent promising new cancer chemotherapeutic agents, since they appear to generate lower quantities of therapeutically unimportant, yet carcinogenic lesions, and more of the therapeutically relevant O6-guanine chloroethylation than the CNUs.

Triapine (3-aminopyridine-2-carboxaldehyde thiosemicarbazone; 3-AP): an inhibitor of ribonucleotide reductase with antineoplastic activity

The enzyme RR catalyzes the conversion of ribonucleoside diphosphates to their deoxyribonucleotide counterparts. RR is critical for the generation of the cytosine, adenine, and guanine deoxyribonucleotide 5'-triphosphate building blocks of DNA, which are present in cells as exceedingly small intracellular pools. Therefore, interference with the function of RR might well result in an agent with significant antineoplastic activity, particularly against rapidly proliferating tumor cells. HUr is the only inhibitor of RR in clinical usage; this agent, however, is a relatively poor inhibitor of the enzyme and has a short serum half-life. Consequently, HUr is a relatively weak anticancer agent. In an effort to develop a more potent inhibitor of RR with utility as an anticancer agent, we have synthesized 3-AP and demonstrated (a) potent inhibition of L1210 leukemia cells in vitro, (b) curative capacity for mice bearing the L1210 leukemia, (c) marked inhibition of RR, and (d) sensitivity of HUr-resistant cells to 3-AP. These findings collectively demonstrate the clinical potential of 3-AP as an antineoplastic agent.

Mitomycin resistance in mammalian cells expressing the bacterial mitomycin C resistance protein MCRA

The mitomycin C-resistance gene, mcrA, of Streptomyces lavendulae produces MCRA, a protein that protects this microorganism from its own antibiotic, the antitumor drug mitomycin C. Expression of the bacterial mcrA gene in mammalian Chinese hamster ovary cells causes profound resistance to mitomycin C and to its structurally related analog porfiromycin under aerobic conditions but produces little change in drug sensitivity under hypoxia. The mitomycins are prodrugs that are enzymatically reduced and activated intracellularly, producing cytotoxic semiquinone anion radical and hydroquinone reduction intermediates. In vitro, MCRA protects DNA from cross-linking by the hydroquinone reduction intermediate of these mitomycins by oxidizing the hydroquinone back to the parent molecule; thus, MCRA acts as a hydroquinone oxidase. These findings suggest potential therapeutic applications for MCRA in the treatment of cancer with the mitomycins and imply that intrinsic or selected mitomycin C resistance in mammalian cells may not be due solely to decreased bioactivation, as has been hypothesized previously, but instead could involve an MCRA-like mechanism.

New insights into the biology and pharmacology of the multidrug resistance protein (MRP) from gene knockout models

Growing interest in the MRP (multidrug resistance protein) gene stems from its importance in multidrug resistance to chemotherapy, its possible use in gene therapy, and its relationship with the glutathione system. The recent generation of mrp gene knockout models in vitro and in vivo is providing information on the mechanism of action and the physiological function(s) of mrp. The importance of mrp in protection of normal tissues from the toxicity of the anticancer agent etoposide has been established. A total block of mrp has been found to be compatible with life, suggesting that MRP inhibitors can be safely used for treating cancer patients. In some sub-classes of leukocytes, mrp contributes to the transport of leukotriene C4, an endogenous glutathione-S-conjugate. However, the baseline expression of mrp does not appear to contribute to the export of glutathione-S-conjugates of alkylating agents, and thus does not exert a protective role against their toxicity. Besides being capable of exporting certain glutathione-S-conjugates, mrp also catalyzes the co-transport of GSH and drug and, presumably, a presently unknown endogenous metabolite(s).

Role of vitamin D3 receptor in the synergistic differentiation of WEHI-3B leukemia cells by vitamin D3 and retinoic acid

WEHI-3B D- cells differentiate in response to 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) but not to all-trans-retinoic acid (RA) or other inducing agents. Combinations of RA with 1,25-(OH)2D3 interact to produce synergistic differentiation of WEHI-3B D- cells. To determine factors involved in the synergistic interaction, expression of the 1,25-(OH)2D3 receptor (VDR) and retinoid receptors, RARalpha and RXRalpha, was measured. No VDR was detected in untreated WEHI-3B D- cells; however, RA and 1,25-(OH)2D3 when used as single agents caused a slight induction of the VDR and in combination produced a marked increase in the VDR. In contrast, no changes in RARalpha and RXRalpha were initiated by these compounds. An RAR-selective agonist combined with 1,25-(OH)2D3 produced synergistic differentiation of WEHI-3B D- cells, whereas an RXR-selective agonist did not. To gain information on the role of the VDR in the synergistic interaction, the VDR gene was transferred into WEHI-3B D+ cells, in which no VDR was detected and no synergism was produced. Expression of the VDR conferred differentiation responsiveness to 1,25-(OH)2D3 in WEHI-3B D+ cells. These findings suggest that (a) induction of VDR expression is a key component in the synergistic differentiation induced by 1,25-(OH)2D3 and RA and (b) RAR and not RXR must be activated for enhanced induction of the VDR and for the synergistic differentiation produced by RA and 1, 25-(OH)2D3.

Functional differentiation signals mediated by distinct regions of the cytoplasmic domain of the granulocyte colony-stimulating factor receptor

Granulocyte colony-stimulating factor receptor (G-CSFR) regulates the proliferation and differentiation of neutrophilic progenitor cells through interaction with its cytokine. Exposure of WEHI-3B D+ myelomonocytic leukemia and myeloid LGM-1 cells overexpressing the G-CSFR to G-CSF resulted in induction of differentiation as measured by (1) the ability to reduce nitroblue tetrazolium (NBT), (2) the expression of Mac-I antigen, and (3) the expression of FcgammaII/III receptor. Mutational analyses indicated that distinct regions of the cytoplasmic domain were critical for efficient induction of each functional marker. The membrane proximal region containing homology sequences of boxes 1 and 2 was important for the activation of all three functional markers of mature neutrophils. Induction of the capacities to express Mac-I antigen or FcgammaII/III receptor also required additional sequences in the membrane proximal region between amino acids 70 and 100 and may be dependent on the phosphorylation of Tyr703. The findings suggest that distinct sequences within the amino-terminal region of the cytoplasmic domain of the receptor are sufficient to induce these functional markers of differentiation, and receptor tyrosine phosphorylation may be necessary.

Formation of a major DNA adduct of the mitomycin metabolite 2,7-diaminomitosene in EMT6 mouse mammary tumor cells treated with mitomycin C

Treatment of EMT6 mouse mammary tumor cells with [3H]mitomycin C (MC) results in the formation of six major DNA adducts, as described earlier using an HPLC assay of 3H-labeled products of enzymatic hydrolysis of DNA isolated from MC-treated cells. Four of these adducts were identified as monofunctional and bifunctional guanine-N2 adducts in the minor groove of DNA. In order to establish relationships between individual types of MC-DNA adducts and biological responses it is necessary to identify all of the adducts formed in cells. To this end we have now identified a predominant, previously unknown adduct formed in MC-treated EMT6 cells as a derivative not of MC, but of 2,7-diaminomitosene (2,7-DAM), the major bioreductive metabolite of MC. Rigorous proof demonstrates that it is a DNA major groove, guanine-N7 adduct of 2,7-DAM, linked at C-10 to DNA. The adduct is relatively stable at ambient temperature, but is readily depurinated upon heating. Its isolation from MC-treated cells indicates that MC is reductively metabolized to 2,7-DAM, which then undergoes further reductive activation to alkylate DNA, along with the parent MC. Low MC:DNA ratios were identified as a critical factor promoting 2,7-DAM adduct formation in an in vitro model calf thymus DNA/ MC/reductase model system, as well as in MC-treated EMT6 cells. The 2,7-DAM-guanine-N7 DNA adduct appears to be relatively noncytotoxic, as indicated by the dramatically lower cytotoxicity of 2,7-DAM in comparison with MC in EMT6 cells. Like MC, 2,7-DAM exhibited slightly greater cytotoxicity to cells treated under hypoxic as compared to aerobic conditions. However, 2,7-DAM was markedly less cytotoxic than MC under both aerobic and hypoxic conditions. Thus, metabolic reduction of MC to 2,7-DAM represents a detoxification process. The differential effects of MC-DNA and 2,7-DAM-DNA adducts support the concept that specific structural features of the DNA damage may play a critical role in the cytotoxic response to a DNA-targeted chemotherapeutic agent.

Choroid plexus epithelial expression of MDR1 P glycoprotein and multidrug resistance-associated protein contribute to the blood-cerebrospinal-fluid drug-permeability barrier

The blood-brain barrier and a blood-cerebrospinal-fluid (CSF) barrier function together to isolate the brain from circulating drugs, toxins, and xenobiotics. The blood-CSF drug-permeability barrier is localized to the epithelium of the choroid plexus (CP). However, the molecular mechanisms regulating drug permeability across the CP epithelium are defined poorly. Herein, we describe a drug-permeability barrier in human and rodent CP mediated by epithelial-specific expression of the MDR1 (multidrug resistance) P glycoprotein (Pgp) and the multidrug resistance-associated protein (MRP). Noninvasive single-photon-emission computed tomography with 99mTc-sestamibi, a membrane-permeant radiopharmaceutical whose transport is mediated by both Pgp and MRP, shows a large blood-to-CSF concentration gradient across intact CP epithelium in humans in vivo. In rats, pharmacokinetic analysis with 99mTc-sestamibi determined the concentration gradient to be greater than 100-fold. In membrane fractions of isolated native CP from rat, mouse, and human, the 170-kDa Pgp and 190-kDa MRP are identified readily. Furthermore, the murine proteins are absent in CP isolated from their respective mdr1a/1b(-/-) and mrp(-/-) gene knockout littermates. As determined by immunohistochemical and drug-transport analysis of native CP and polarized epithelial cell cultures derived from neonatal rat CP, Pgp localizes subapically, conferring an apical-to-basal transepithelial permeation barrier to radiolabeled drugs. Conversely, MRP localizes basolaterally, conferring an opposing basal-to-apical drug-permeation barrier. Together, these transporters may coordinate secretion and reabsorption of natural product substrates and therapeutic drugs, including chemotherapeutic agents, antipsychotics, and HIV protease inhibitors, into and out of the central nervous system.

Hypoxia-selective nitrobenzyloxycarbonyl derivatives of 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)hydrazines

Some 4- and 2-(nitrobenzyloxycarbonyl)-1, 2-bis(methylsulfonyl)-1-(2-chloroethyl)hydrazines (4, 6, and 7) were synthesized and evaluated for their ability to exert preferential toxicity to hypoxic EMT6 mammary carcinoma cells using a colony-forming assay. Of these, the 4,5-dimethoxy-2-nitro analogue 6 (50 microM, 1-h exposure) caused greater than 3 logs of kill of hypoxic cells, with relatively minor toxicity to corresponding aerobic cells. The ability of 4-nitro (4) and 4,5-dimethoxy-2-nitro (6) analogues to reach and kill hypoxic cells of solid tumors was also demonstrated using intradermally implanted EMT6 solid tumors in mice. In addition, a possible source of toxicity to normal tissue, i. e., the activation of the 4-nitrobenzyl derivative 4 by glutathione S-transferase-catalyzed thiolysis, was essentially eliminated by replacing one of the benzylic methylene protons by a methyl group. The 4-nitro (4) and 4,5-dimethoxy-2-nitro (6) analogues also appear to be reduced more easily under acidic conditions (pH 6.0) than under neutral conditions, as measured by differential pulse polarography. Since the pH in hypoxic regions is often lower than that in adjacent aerobic regions, this property should aid in the cytotoxic action of these agents against hypoxic cells of solid tumors.

Synthesis of 2'-methylene-substituted 5-azapyrimidine, 6-azapyrimidine, and 3-deazaguanine nucleoside analogues as potential antitumor/antiviral agents

2'-Deoxy-2'-methylene-6-azauridine (5) and 2'-deoxy-2'-methylene-6-azacytidine (8) have been synthesized via a multi-step procedure from 6-azauridine. 2'-Deoxy-2'-methylene-5-azacytidine (14a) and 2'-deoxy-2'-methylene-3-deazaguanosine (19a) and their corresponding alpha-anomers (14b and 19b) have been synthesized by the transglycosylation of 3',5'-O-(1,1,3,3- tetraisopropyldisiloxane-1,3-diyl)-2'-deoxy-2'-methyleneu ridine (12) with silylated 5-azacytosine and silylated N2-palmitoyl-3-deazaguanine, respectively, in the presence of trimethylsilyl trifluoromethanesulfonate as the catalyst in anhydrous dichloroethane, followed by separation of the isomers and deprotection of the blocking groups. These compounds were tested for cytotoxicity against B16F10, L1210, and CCRF-CEM tumor cell lines and for antiviral activity against HIV-1, HSV-1, and HSV-2.

Coinduction of embryonic and adult-type globin mRNAs by sodium butyrate and trichostatin A in two murine interleukin-3-dependent bone marrow-derived cell lines

Using an RNase protection assay, globin mRNA species expressed in clones derived from Ba/F3 and B6SUtA cells transfected with the erythropoietin receptor (EpoR) and selected with erythropoietin (Epo) were compared with globin mRNA species induced in corresponding parental cells by sodium butyrate (SB) and trichostatin A (TSA). betaMajor/betaminor- and -1/-2-globin mRNAs were the major species, with trace amounts of epsilon-globin mRNA, formed in Epo-stimulated EpoR+ Ba/F3 clones, whereas SB and TSA allowed expression of all species of globin mRNAs, ie, epsilon, betah1, betamajor/betaminor, zeta, and -1/-2, in parental Ba/F3 cells. In contrast, epsilon- and -1/-2-globin mRNAs were the major species present in Epo-stimulated EpoR+ B6SUtA clones, whereas SB and TSA activated epsilon-, betah1-, betaS/betaT-, and -1/-2-globin genes in parental B6SUtA cells; zeta-globin mRNA was not detected in SB- and TSA-treated B6SUtA cells. Because TSA is a specific inhibitor of histone deacetylase, the mimicry of action exhibited by SB and TSA suggests that the effects of SB are mediated through its ability to inhibit histone deacetylase and that histone deacetylase is an integral part of the repression of globin genes in these interleukin-3-dependent cells. Efficient coinduction of embryonic and adult types of globin mRNA in bone marrow cell lines derived from adult mice indicates that adult hematopoietic precursors possess an embryonic nature. These cell lines are useful models to study the mechanism(s) of developmental globin gene switching.

Role of gp55 in restoring the sensitivity of Friend murine erythroleukemia cells to erythropoietin by exposure to dimethyl sulfoxide

Although Friend murine erythroleukemia (MEL) cells express the erythropoietin receptor (EpoR), they are insensitive to erythropoietin (Epo). The nonresponsiveness to Epo presumably results from gp55, the product of the env gene encoded by the Friend spleen focus-forming virus (F-SFFV), acting as a pseudoligand and constitutively activating the receptor. Dimethyl sulfoxide (DMSO) induced the differentiation of MEL cells and partially restored responsiveness to Epo, with both increased proliferation and further hemoglobin synthesis. Treatment of MEL cells with DMSO caused a decrease in the cellular content of gp55 as measured by Western analysis and an increase in the level of the EpoR as measured by [125I]Epo binding. These changes were produced at least in part at the transcriptional level, because DMSO treatment caused a decrease and an increase in the levels of the mRNAs for gp55 and EpoR, respectively. To ascertain the role of gp55 in the restoration of the sensitivity of MEL cells to Epo by exposure to DMSO, expression vectors containing gp55 DNA in the sense and antisense orientations were transfected into MEL cells to increase or decrease, respectively, the amount of cellular gp55. An increase in the level of gp55 interfered with the ability of DMSO to restore sensitivity to Epo, whereas a decrease in the level of gp55 increased the Epo-sensitizing effects of DMSO. [125I]Epo was chemically cross-linked to a component with a calculated molecular weight of 65 kDa. DMSO treatment caused an increase in the level of [125I]Epo cross-linking. The protein cross-linked to Epo was immunoprecipitated with anti-EpoR serum but not with anti-gp55 serum, suggesting that Epo was cross-linked to its receptor. The finding of a decrease in the cellular content of gp55, an increase in the level of the EpoR, and an increase in the formation of the Epo/EpoR complex is consistent with the acquisition of sensitivity to Epo by MEL cells following treatment with DMSO.

Exploring the mechanistic aspects of mitomycin antibiotic bioactivation in Chinese hamster ovary cells overexpressing NADPH:cytochrome C (P-450) reductase and DT-diaphorase

We have directly demonstrated the involvement of human NADPH: cytochrome c (P-450) reductase in the aerobic/hypoxic differential toxicity of mitomycin C and porfiromycin in living cells by varying only this enzyme in a transfected cell line. In the same manner, we have implicated rat DT-diaphorase in the aerobic and hypoxic activation of mitomycin C, but found only a minor role for this enzyme in the aerobic activation of porfiromycin. DT-Diaphorase does not cause the production of an aerobic/hypoxic differential toxicity by mitomycin C, but rather activates this agent through an oxygen insensitive pathway. The evidence suggests that DT-diaphorase activates mitomycin C more effectively than porfiromycin, with porfiromycin being preferentially activated through a one-electron reductive pathway. The therapeutic potential of mitomycin antibiotics in the treatment of cancer can be envisioned to be enhanced for those tumors containing elevated levels of the bioreductive enzymes. However, cytogenetic heterogeneity within the tumor cell population and the various environmental factors which impact on bioreductive enzyme function, including pH and oxygen tension, may subvert this approach. Moreover, if high tumor levels of a drug activating enzyme reflect high levels in the normal tissues of the patient, normal tissue damage may also be enhanced with possibly no improvement in the therapeutic ratio. Approaches utilizing gene therapy, whereby a specific bioreductive catalyst is introduced into the tumor cell population via a targeting vehicle to activate a particular prodrug, may be more effective in that not only will the prodrug of choice be specifically activated in the tumor, but the source of the catalyst, be it bacterial, rodent, or human, will not be important. In fact, in the case of DT-diaphorase and mitomycin C, the rat form of the enzyme could be advantageous because it is more effective in activating mitomycin C than is the human form of this enzyme. Assuming targeted gene delivery to malignant cells, a non-host enzyme which is more effective at activating mitomycin C than the analogous host enzyme might also result in less drug activation in normal tissue and, hence, less normal tissue toxicity.

Characterization of the domains of the erythropoietin receptor necessary for induction of cell growth and differentiation

To define the cytoplasmic region(s) of the erythropoietin receptor (EpoR) necessary for promotion of growth and induction of differentiation, mutated EpoR cDNAs containing truncations and conversions of tyrosine residues to phenylalanines were generated. Constructs were introduced into IL-3 dependent Ba/F3 cells by electroporation, and individual transfectants were propagated in methylcellulose-containing medium in the presence of erythropoietin (Epo). Truncated receptor at amino acid 350 was partially deficient in promoting cellular growth and the receptor lacking the box 2 region was not capable of inducing cellular growth. When a large number of cells were screened in Epo-containing liquid medium, clones arose in which Epo-dependent growth was due to activation of the endogenous EpoR gene. Analysis of the degree of Epo-dependent differentiation of the transfectants, based on the steady-state levels of beta major-globin mRNA, showed that the carboxyl terminal 133 amino acids and tyrosyl residues located at positions 429/431 and 460/464 were not necessary for the induction of differentiation. Examination of clones from diverse origins demonstrated that an inverse relationship existed between the rate of Epo-induced cellular replication and the degree of Epo-induced differentiation.

Regulation of the expression of enzymes involved in the replication of DNA in chemically-induced granulocytic differentiation of HL-60 leukemia cells

The expression of seven enzymes involved in the biosynthesis of DNA was measured in HL-60 promyelocytic leukemia cells treated with dimethylsulfoxide (DMSO) or all-trans retinoic acid (RA) to gain information on their role in the termination of proliferation in cells undergoing granulocytic differentiation. The steady-state levels of the mRNAs for topoisomerase I, topoisomerase II. DNA polymerase-alpha, thymidylate synthase, thymidine kinase and hypoxanthine-guanine phosphoribosyltransferase progressively declined from day 3 to day 7 of exposure to the polar solvent or the retinoid suggesting that the expression of these enzymes is coordinately regulated. In contrast, a pronounced difference between the two inducers of differentiation occurred in the expression of the mRNA of the M2 subunit of ribonucleotide reductase, with DMSO causing virtually complete inhibition of the expression of the M2 subunit of the enzyme from day 5 through day 7, with no change in the steady-state levels of the mRNA being produced by retinoic acid. Measurement of the enzymatic activities of two of these catalysts, thymidylate synthase and thymidine kinase, in cells exposed to the two inducers of maturation corroborated the findings at the level of the mRNAs, with corresponding decreases in the activity of these enzymes. The findings collectively demonstrate that the down-regulation of the expression of a relatively wide variety of enzymes involved in DNA replication occurs as late events in the granulocytic differentiation of HL-60 cells, ensuring that cellular replication cannot occur in terminally differentiated cells.

Regulation of the expression of enzymes involved in the replication of DNA in chemically induced monocytic/macrophagic differentiation of HL-60 leukemia cells

The expression of a number of housekeeping enzymes of DNA biosynthesis was measured in HL-60 promyelocytic leukemia cells undergoing monocytic/macrophagic differentiation following treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA) or 1alpha,25-dihydroxyvitamin D3 (vitamin D3). Progressive decreases in the steady-state levels of the mRNAs for thymidylate synthase, topoisomerase II, and hypoxanthine guanine phosphoribosyltransferase occurred following exposure to TPA or vitamin D3. In contrast, the steady-state levels of the mRNAs for thymidine kinase, topoisomerase I, and DNA polymerase-alpha did not decrease until days 3-5 of treatment with vitamin D3 and then progressively declined thereafter. The mRNAs for thymidine kinase and topoisomerase I decreased slightly and the mRNA for DNA polymerase-alpha by 30-40%, and then remained constant between days 1 to 3 of treatment with the phorbol ester. The M2 subunit of ribonucleotide reductase exhibited an even greater difference, with no change in the steady-state concentration of mRNA over 3 days of exposure to TPA or vitamin D3. On days 5-7 of treatment with vitamin D3, essentially complete loss of the expression of the mRNA for the M2 subunit of ribonucleotide reductase occurred. Measurement of the enzymatic activities of thymidylate synthase and thymidine kinase in cells exposed to either of the inducers of maturation corroborated the findings at the level of the mRNAs, with corresponding decreases in the activity of these enzymes. The results indicate that the down-regulation of the expression of housekeeping enzymes of DNA replication occurs as late events in HL-60 cells undergoing monocytic/macrophagic differentiation, implying that the decreases in their gene expression are the result of the termination of proliferation rather than an initiating event in the cessation of DNA biosynthesis.

Identification of a repressor of the differentiation of WEHI-3B D- leukemia cells

The WEHI-3B D+ leukemia is a near-diploid differentiation-competent cell line that undergoes myeloid differentiation in response to retinoic acid. WEHI-3B D- cells, derived from WEHI-3B D+ cells, are near tetraploid and not responsive to the differentiation-inducing properties of the retinoid. To gain information on mechanisms that regulate the maturation of these two cell lines, several multiploid cell lines have been established through fusion of WEHI-3B D+ and WEHI-3B D- cells. Studies with the multiploid cell lines have shown that (a) the cellular growth rate decreases with increased DNA ploidy; (b) near-tetraploid D+/+ cells, obtained by fusing WEHI-3B D+ with WEHI-3B D+ cells, remain differentiation-competent, demonstrating that no direct relationship exists between differentiation competency and DNA ploidy; and (c) near-hexaploid D +/- and D -/+ cells, formed by fusion of WEHI-3B D+ with WEHI-3B D- cells, do not respond to differentiation inducers, suggesting the inhibition of the differentiation machinery of WEHI-3B D+ cells by components from maturation-incompetent WEHI-3B D- cells. The scl transcription factor gene is expressed in WEHI-3B D- cells and is absent in WEHI-3B D+ cells. Overexpression of scl by transfection of scl cDNA in WEHI-3B D+ cells markedly decreased the capacity of retinoic acid to induce differentiation, suggesting that scl functions as a repressor of differentiation in WEHI-3B cell lines.

The intracellular location of NADH:cytochrome b5 reductase modulates the cytotoxicity of the mitomycins to Chinese hamster ovary cells

NADH:cytochrome b5 reductase activates the mitomycins to alkylating intermediates in vitro. To investigate the intracellular role of this enzyme in mitomycin bioactivation, Chinese hamster ovary cell transfectants overexpressing rat NADH:cytochrome b5 reductase were generated. An NADH:cytochrome b5 reductase-transfected clone expressed 9-fold more enzyme than did parental cells; the levels of other mitomycin-activating oxidoreductases were unchanged. Although this enzyme activates the mitomycins in vitro, its overexpression in living cells caused decreases in sensitivity to mitomycin C in air and decreases in sensitivity to porfiromycin under both air and hypoxia. Mitomycin C cytotoxicity under hypoxia was similar to parental cells. Because NADH:cytochrome b5 reductase resides predominantly in the mitochondria of these cells, this enzyme may sequester these drugs in this compartment, thereby decreasing nuclear DNA alkylations and reducing cytotoxicity. A cytosolic form of NADH:cytochrome b5 reductase was generated. Transfectants expressing the cytosolic enzyme were restored to parental line sensitivity to both mitomycin C and porfiromycin in air with marked increases in drug sensitivity under hypoxia. The results implicate NADH:cytochrome b5 reductase in the differential bioactivation of the mitomycins and indicate that the subcellular site of drug activation can have complex effects on drug cytotoxicity.

Enhancement by antisense oligonucleotides to NF-kappaB of the differentiation of HL-60 promyelocytic leukemia cells induced by vitamin D3

We have demonstrated previously that a phosphorothioate antisense oligonucleotide to the p65 subunit of the inducible transcription factor NF-kappaB produced rapid changes in the expression of leukocyte integrin CD11b (Mo 1) and in the adhesion of dimethylsulfoxide (DMSO)-differentiated HL-60 cells stimulated by 12-O-tetradecanoylphorbol 13-acetate. We have also shown that a variety of agents which inhibit NF-kappaB, including vitamin E and related antioxidants, curcumin and several non-steroidal anti-inflammatory agents, significantly enhanced the differentiation of HL-60 leukemia cells when combined with low levels of 1,25-dihydroxyvitamin D3 (vitamin D3). To provide further evidence that interference with the activation of NF-kappaB affects the maturation of HL-60 leukemia cells by creating an environment conducive to terminal differentiation, we measured the effects of phosphorothioate antisense oligonucleotides to the various subunits of NF-kappaB on the differentiation of HL-60 cells produced by low levels of vitamin D3. When used alone these oligonucleotides had no significant effect on the differentiation of HL-60 cells. However, the antisense oligomer to the Rel A subunit of NF-kappaB markedly increased the extent of differentiation produced by low levels of vitamin D3. An enhancement of the differentiation of HL-60 cells induced by vitamin D3 was also obtained by several transcription factor decoys designed to mimic the consensus sequences of genes activated by Rel A. The findings provide additional support for the concept that inhibition of the activation of NF-kappaB may be involved in regulating the entry of promyelocytic leukemia cells into a differentiation pathway.

Induction of the differentiation of HL-60 promyelocytic leukemia cells by nonsteroidal anti-inflammatory agents in combination with low levels of vitamin D3

Previous experiments have shown that a variety of agents that interfere with the activity of the transcription factor NF-kB significantly enhanced the differentiation of HL-60 leukemia cells when combined with low levels of the monocytic/macrophagic differentiating agent vitamin D3. These include an antisense phosphorothioate oligonucleotide to the Rel A subunit of NF-kB, vitamin E and other antioxidants, and curcumin. Acetylsalicylic acid and other nonsteroidal anti-inflammatory agents represent another group of agents that have been reported to inhibit NF-kB at serum levels approximating those obtained during long-term therapy of chronic inflammatory states. To determine whether nonsteroidal anti-inflammatory agents also were capable of enhancing the differentiation of HL-60 leukemia cells produced by vitamin D3, we measured the effects of a variety of nonsteroidal anti-inflammatory agents on the maturation of HL-60 cells produced by low levels of vitamin D3. Acetylsalicylic acid by itself had no significant effect on the differentiation of HL-60 cells; however, this agent markedly increased the degree of differentiation produced by low levels of vitamin D3. Furthermore, a variety of other nonsteroidal anti-inflammatory agents of different chemical classes exhibited similar enhancements of the maturation of HL-60 cells when combined with vitamin D3. An analogous increase in the differentiation of HL-60 cells was also obtained by combination of several nonsteroidal anti-inflammatory agents with the granulocytic inducing agent, retinoic acid, but not with dimethylsulfoxide. The nonsteroidal anti-inflammatory agents also enhanced the differentiation of HL-60 cells when combined with vitamin D analogs which share the receptor binding properties of vitamin D3; however, a vitamin D analog which caused significant calcium mobilization, but was less effective in receptor binding than vitamin D3, did not induce the differentiation of HL-60 cells in the presence or absence of anti-inflammatory agents. The findings suggest that the nonsteroidal anti-inflammatory agents may have utility in the treatment of acute promyelocytic leukemia when used with the D vitamins or retinoic acid.

Bioreductive alkylating agent porfiromycin in combination with radiation therapy for the management of squamous cell carcinoma of the head and neck

Porfiromycin (methyl mitomycin C) has been shown in laboratory studies to have increased preferential cytotoxicity to hypoxic cells and therefore may provide enhanced therapeutic efficacy over mitomycin C when used in combination with radiation therapy (RT). The purpose of the two clinical studies reported here is to evaluate the concomitant use of porfiromycin with RT in the management of squamous cell carcinoma of the head and neck. Between October 1989 and July 1992, 21 patients presenting with locally advanced stage III/IV squamous cell carcinoma of the head and neck were entered into a phase I toxicity trial evaluating porfiromycin as an adjunct to RT. Patients were eligible if they had biopsy documented squamous cell carcinoma of the head and neck with a low probability of cure by conventional means. Patients were treated with standard fractionated daily RT to a total median dose of 63 Gy, with porfiromycin administered on days 5 and 47 of the course of RT. Upon completion of this phase I trial, a phase III trial was initiated in November 1992 randomizing patients with squamous cell carcinoma of the head and neck to RT with mitomycin C vs. RT with porfiromycin. There is no radiation only arm in this current trial. To date, 75 patients have been entered on this trial and acute toxicity data are available on 67 patients (34 porfiromycin, 31 mitomycin C) who have completed their entire course of treatment. Median follow-up of the 21 patients enrolled in the phase I porfiromycin trial is 58.5 months. Of the 21 patients, 5 were treated at a dose of 50 mg/M2, 4 at 45 mg/M2, and the final 12 at 40 mg/M2, which appeared to result in acceptable acute hematological and nonhematological toxicities. As of December 1995, 14 of the 21 patients have died with disease and 7 remain alive and free of disease, resulting in a 5-year actuarial survival of 32%. Of the patients enrolled to date in the phase III randomized trial of mitomycin C vs. porfiromycin, there have been no statistically significant differences between the two arms with respect to white blood cell count (WBC), platelet, or hemoglobin nadirs. Acute nonhematological toxicities including mucositis, epidermitis, odynophagia, and nausea have also been comparable. Two patients in this current randomized trial died during treatment, apparently of nondrug-related causes. We conclude that the bioreductive alkylating agent porfiromycin has demonstrated an acceptable toxicity profile to date. Final analysis of the phase I trial, which revealed a 5-year no evidence of disease survival rate of 32% in patients with locally advanced disease and a low probability of cure, appears encouraging. We anticipate completion of the current ongoing trial comparing mitomycin C to porfiromycin in the next 2 years. Further investigations, including large-scale multiinstitutional trials employing bioreductive alkylating agents or other hypoxic cell cytotoxins as adjuncts to RT, are warranted.

Evidence that the multidrug resistance protein (MRP) functions as a co-transporter of glutathione and natural product toxins

The MRP (multidrug resistance protein) gene, a member of the ubiquitous superfamily of ATP-binding cassette transporters, is associated with the multidrug resistance of mammalian cells to natural product anticancer agents. We have previously shown that abrogation of MRP expression by gene targeting leads to hypersensitivity to several drugs. In two independently produced MRP double knockout clones, the baseline export of glutathione (GSH) was one-half that of wild-type embryonic stem (ES) cells. The export of GSH from wild-type ES cells, but not from the MRP double knockout clones, increased in the presence of etoposide (VP-16) and sodium arsenite, accompanied by equivalent decreases in intracellular levels of GSH. In the two MRP double knockout clones, the intracellular steady-state concentration of etoposide was twofold greater than that in wild-type cells. Depletion of intracellular GSH by D,L-buthionine sulfoximine increased the intracellular accumulation of radiolabeled etoposide in parental ES cells up to the level present in the two MRP knockout clones but did not change etoposide levels in the MRP knockout clones. These observations provide evidence that: (a) MRP exports GSH physiologically, presumably in association with an endogenous compound(s); (b) baseline MRP expression protects cells from the toxic effects of xenobiotics by effluxing the xenobiotics and GSH from the intracellular compartment into the extracellular medium by a co-transport mechanism; and (c) disruption of the gene encoding MRP abrogates the cotransport of xenobiotics and GSH.

Disruption of the murine MRP (multidrug resistance protein) gene leads to increased sensitivity to etoposide (VP-16) and increased levels of glutathione

The mrp (multidrug resistance protein) gene has been associated with the multidrug resistance of cancer cells in vitro and in vivo. To gain information on its physiological role, embryonic stem cells were used to generate mice homozygous for a disruption of the mrp gene, resulting in complete abrogation of mrp expression. No physiological abnormalities were observed, at least up to 4 months of age. Viability, fertility, and a range of histological, hematological, and serum-chemical parameters were similar in mrp(+/+) and mrp(-/-) mice. mrp(-/-) mice displayed an increased sensitivity to etoposide phosphate (2-fold) accompanied by greater bone marrow toxicity, whereas the acute toxicity of sodium arsenite was equivalent in mrp(+/+) and mrp(-/-) mice. Tissue levels of glutathione (GSH) were elevated in breast, lung, heart, kidney, muscle, colon, testes, bone marrow cells, blood mononuclear leukocytes, and blood erythrocytes of mrp(-/-) mice and were unchanged in organs known to express little if any mrp, such as the liver and small intestine. The increase in GSH was not due to an increase in the activity of gamma-glutamylcysteine synthetase, the rate-limiting enzyme for GSH synthesis. The findings demonstrate that mrp is dispensable for development and growth but exerts a role in drug detoxification and GSH metabolism.